Network Steganography

7:00 p.m., Shanghai

An employee of an electronic equipment factory uploads a music file to an online file-sharing site. Hidden in the MP3 file (Michael Jackson's album Thriller) are schematics of a new mobile phone that will carry the brand of a large American company. Once the employee's Taiwanese collaborators download the file, they start manufacturing counterfeit mobile phones essentially identical to the original?even before the American company can get its version into stores.

3:30 p.m., somewhere in Afghanistan

A terrorist hunted by the U.S. Federal Bureau of Investigation posts an excerpt from the motion picture High School Musical Three: Senior Year on Facebook. Inside are hidden instructions for a bomb attack on a commuter rail line in southern Europe. Later that day, terrorists based in Athens follow the instructions to plan a rush hour attack that kills hundreds of people.

4:00 a.m., Malibu, Calif.

A very famous actor (VFA) has a brief conversation with a well-known director (WKD) over Skype, an application that lets them make free voice calls over the Internet. They discuss the medical problems of VFA's cat in great detail. When the conversation is over, WKD's computer has a sleazy new addition?in a folder on his desktop, there is a picture of a nude teenager, along with her mobile number and the date and time at which WKD will meet her at VFA's pool party for a photo session.

What all these scenarios have in common is an information-smuggling technique called steganography?the communication of secret messages inside a perfectly innocent carrier. Think of steganography as meta-encryption: While encryption protects messages from being read by unauthorized parties, steganography lets the sender conceal the fact that he has even sent a message. After the 11 September attacks in 2001, rumors flew that they had been carried out with some help from steganography. A 2001 New York Times article described fake eBay listings in which routinely altered pictures of a sewing machine contained malevolent cargo. The link to 9/11 was never proved or disproved, but after those reports, the interest in steganographic techniques and their detection greatly increased.

Steganography use is on the rise, and not just among criminals, hackers, child pornographers, and terrorists. Persecuted citizens and dissidents under authoritarian regimes use it to evade government censorship, and journalists can use it to conceal sources. Investigators even use it on occasion to bait and trap people involved in industrial espionage: In the 1980s, to trace press leaks of cabinet documents, British Prime Minister Margaret Thatcher had government word processors altered to encode a specific user identity in the spaces between words. When leaked material was recovered, the identity of the leaker could be established by analyzing the pattern of those spaces.

Steganography is evolving alongside technology. A few years ago the cutting edge in steganographic tools involved hiding messages inside digital images or sound files, known as carriers, like that Thriller MP3. The technique quickly evolved to include video files, which are relatively large and can therefore conceal longer messages.

Now steganography has entered a new era, with stupendously greater potential for mischief. With the latest techniques, the limitations on the length of the message have basically been removed. Consider our example involving the use of Skype. Whereas the first two examples each required a carrier?an MP3 song and a video?there was no such requirement for the transmission of that nude photo. The data were secreted among the bits of a digital Voice over Internet Protocol conversation. In this new era of steganography, the mule that coconspirators are using is not the carrier itself but the communication protocols that govern the carrier's path through the Internet. Here's the advantage: The longer the communicators talk, the longer the secret message (or more detailed the secret image) they can send. Most strikingly, the concealment occurs within data whose inherent ephemerality makes the hidden payload nearly impossible to detect, let alone thwart.